Proton conducting materials have a number of applications. Proton conducting membranes are widely utilized in devices which use a chemical reaction to produce or store electricity, or use electricity to drive a chemical process. Materials which conduct both protons and electrons (“mixed proton and electron conductors”) are utilized in related applications.
Electrochemical devices depend on the flow of protons, or the flow of both protons and electrons through a proton conducting membrane. Exemplary electrochemical devices include a fuel cell, an electrolysis cell, a hydrogen separation cell, a battery, a supercapacitor, and a membrane reactor. There are other electrochemical devices which also use a proton conducting membrane.
An important use for proton conducting membranes is in fuel cells. Fuel cells are attractive alternatives to combustion engines for the generation of electricity because of their higher efficiency and the lower level of pollutants they produce. A fuel cell generates electricity from the electrochemical reaction of a fuel e.g. methane, methanol, gasoline, or hydrogen, with oxygen normally obtained from air.
There are three common types of fuel cells used at temperatures close to ambient. A direct hydrogen/air fuel cell system stores hydrogen and then delivers it to the fuel cell as needed.
In an indirect hydrogen/air fuel cell, hydrogen is generated on site from a hydrocarbon fuel, cleaned it of carbon monoxide (CO), and subsequently fed to the fuel cell.
A direct methanol fuel cell (“DMFC”), feeds methanol/water solution directly to the fuel cell, e.g., without any fuel processing. One type of DMFC has been described, for example, in U.S. Pat. No. 5,559,638. There are various advantages and disadvantages inherent within all three configurations. All are, to a greater or lesser extent, limited by the performance of the proton conducting membrane.
Nafion™, a perfluorinated sulphonic acid polymer, is often used as a membrane material for fuel cells which operate at temperatures close to ambient. Other hydrated polymers have also been used as proton conductive materials. Membranes of modified perfluorinated sulfonic acid polymers, polyhydrocarbon sulfonic acid polymers, and composites thereof are also known. These and related polymers are used in hydrated form. Proton transport occurs by the motion of hydronium ions, H3O+. Water is necessary in order to facilitate proton conduction. Loss of water immediately results in degradation of the conductivity. Moreover, this degradation is irreversible—a simple reintroduction of water to the system does not restore the conductivity. Thus, the electrolyte membranes of these hydrated polymer-based fuel cells must be kept humidified during operation. This introduces a host of balance-of-plant needs for water recirculation and temperature control.
A second limitation derives from the need to maintain water in the membrane. In order to maintain hydration, the temperature of operation cannot exceed 100° C. without cell pressurization. High temperature operation could be desirable, however, to increase catalyst efficiency in generating protons at the anode (in both H2 and direct methanol fuel cells) and to improve catalyst tolerance to carbon monoxide (“CO”). CO is often present in the fuel that is used in the fuel cells. The CO can poison the precious metal catalysts. This is particularly problematic in indirect hydrogen/air fuel cells for which hydrogen is generated on site. High temperatures also benefit the reduction reaction on the cathode.
Another limitation of hydrated polymer electrolytes occurs in applications in methanol fuel cells. These polymers can be permeable to methanol. Direct transport of the fuel (i.e. methanol) across the membrane to the air cathode results in losses in efficiency.
Alternate proton conducting materials, which do not require humidification, which can be operated at slightly elevated temperatures, and/or which are impermeable to methanol, are desirable for fuel cell applications.
In the field of hydrogen separation, a proton conducting membrane is utilized to separate hydrogen from other gases such as CO and/or CO2. Palladium is often used for this application. Palladium is permeable to molecular hydrogen, but not in general to other gases. There are drawbacks to the use of this material. It is expensive and the hydrogen diffusion rate is low. It would be desirable to develop new materials which are less expensive and exhibit higher proton/hydrogen transport rates.
In general, materials utilized in other electrochemical devices such as electrolysis cells, batteries, supercapacitors, etc., include liquid acid electrolytes, which are highly corrosive, and solid polymer proton conductors, which require humidification or exhibit insufficient proton conductivity. High conductivity, high chemical and thermal stability solid membranes with good mechanical properties are desirable for all of these applications.